Optical fiber mounting waveguide device and method for fabricating same

ABSTRACT

An optical fiber mounting waveguide device includes a substrate, an optical fiber mounting groove provided on a part of the substrate for mounting an optical fiber, an under cladding layer and a core sequentially formed on the substrate, and an over cladding layer formed on the core, the over cladding layer having an end surface facing to the optical fiber mounting groove, and wherein the core and the under cladding layer are protruded toward the optical fiber mounting groove with respect to the end surface of the over cladding layer.

The present application is based on Japanese Patent Application No.2007-123470, the entire contents of which are incorporated herein byreference.

The present Application is a Divisional Application of U.S. patentapplication Ser. No. 12/149,530, filed on May 2, 2008 now U.S. Pat. No.7,876,988.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber mounting waveguidedevice and a method for fabricating the same in which an opticalconnection loss is small, and productivity is high.

2. Related Art

Conventionally, in a waveguide device on which an optical fiber ismounted (hereinafter, referred as “optical fiber mounting waveguidedevice”), an optical fiber groove for guiding an optical fiber on anextended line of a core and easily fixing the optical fiber stably.

The optical fiber mounting waveguide device is provided with endsurfaces of a cladding and the core facing to the optical fiber mountinggroove. In other words, the cladding is provided with a stepped portion,and a bottom part of the stepped portion is provided as a bottom surfaceof the optical fiber groove, so that an end surface of the optical fibermounted in the optical fiber groove contacts with the end surfaces ofthe cladding and the core.

Japanese Patent Application Laid-Open No. 2006-184754 (JP-A-2006-184754)and Japanese Patent Application Laid-Open No. 2002-267860(JP-A-2002-267860) disclose the conventional optical fiber mountingwaveguide devices.

FIG. 4 is a plan view of an optical fiber mounting waveguide device in afirst conventional example.

As shown in FIG. 4, the conventional optical fiber mounting waveguidedevice 101 comprises an optical fiber groove 102 for mounting an opticalfiber 105, a cladding 103, a core 104 surrounded by the cladding 103 andformed until an end surface of the cladding 103, in which an end surfaceof the optical fiber 105 mounted in the optical fiber groove 102contacts with the end surface of the cladding 103, so that a core (notshown) of the optical fiber 105 is optically coupled to the core 104 inthe optical fiber mounting waveguide device 101.

FIG. 5 is a partially enlarged view of the conventional optical fibermounting waveguide device shown in FIG. 4.

FIG. 5 shows an enlarged plan view of a portion in vicinity of the endsurface of the cladding 103. A tip portion 104 a of the core 104 hasroundness as shown in FIG. 5. The roundness of the end surface of thecladding 103 is formed as follows. For example, when the optical fibermounting waveguide device 101 is fabricated by photolithographytechnique (direct exposure), a photomask is provided for the exposure ona base waveguide substrate so that the core 104 and the cladding 103 areformed to be defined in a desired configuration on the base waveguidesubstrate. At this time, for example, a diffraction of light may occurat edges of the photomask, so that a mask pattern that is rightangle-edged on the photomask is exposed with the roundness. Even if acore pattern in the photomask is right angle-edged, since the endsurface of the cladding 103 is located at the edge of the photomask, thetip portion 104 a of the formed core 104 has the roundness. Inparticular, the roundness is easily formed at the tip portion 104 a ofthe core 104, when a core diameter (core size) is small.

In addition, a space between the tip portion 104 a having the roundnessof the core 104 and an end surface 103 a of the cladding 103 is filledwith the cladding 103.

When the tip portion 104 a of the core 104 has the roundness as shown inFIG. 5, the cladding 103 intrudes a gap between a curved surface of thetip portion 104 a of the core 104 and the optical fiber 105, so that aninterface between the core 104 and cladding 103 is curved. As a result,an optical path is changed due to the refraction of the light, and thelight significantly leaks in adjacent cores, so that an opticalisolation is deteriorated and an optical connection loss is increased.

In addition, when the tip portion 104 a of the core 104 has theroundness and is covered by the cladding 103, an optical couplingproperty is greatly changed in accordance with a relative position (aposition in an orientation along the end surface) of the core 104 withrespect to the optical fiber 105, so that a dispersion in the loss dueto variation of the relative position (the orientation along the endsurface) between the core tip portion 104 a and the optical fiber causedby manufacturing dispersion. As a result, the productivity of theoptical fiber mounting waveguide device is decreased.

FIG. 6 is a cross sectional view of an optical fiber mounting waveguidedevice in a second conventional example.

In the optical waveguide device disclosed by JP-A-2002-267860, there isa problem in that the core end surface has the roundness as describedabove so that the optical connection loss is increased in the connectionwith the optical fiber. Further, as shown in FIG. 6, according to thismethod, an optical connection end surface of a core 61 is formed in aconcave portion of a cladding 62. Thereafter, a cladding material and asubstrate covering the optical connection end surface are removed toprovide an optical waveguide in which the core 61 is protruded from acladding end surface. Therefore, there is a problem in that the core 61is damaged during the removal process of the cladding 62.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to solve theproblem, and to provide an optical fiber mounting waveguide device and amethod for fabricating the same in which the optical connection loss issmall and the productivity is high.

According to a first feature of the invention, an optical fiber mountingwaveguide device comprises:

a substrate;

an optical fiber mounting groove provided on a part of the substrate formounting an optical fiber;

an under cladding layer and a core sequentially formed on the substrate;and

an over cladding layer formed on the core, the over cladding layerhaving an end surface facing to the optical fiber mounting groove, and

wherein the core and the under cladding layer are protruded toward theoptical fiber mounting groove with respect to the end surface of theover cladding layer.

According to a second feature of the invention, an optical fibermounting waveguide device comprises:

a substrate;

an optical fiber mounting groove provided on the substrate for mountingan optical fiber;

an under cladding layer and a core sequentially formed on the substrate;and

an over cladding layer formed on the core, the over cladding layerhaving an end surface facing to the optical fiber mounting groove, and

wherein the core has a protruded part extending toward the optical fibermounting groove with respect to the end surface of the over claddinglayer and a downwardly protruded part.

In the optical fiber mounting waveguide device, the core may comprise aplurality of cores coupled to each other outside the end surface of theover cladding layer.

In the optical fiber mounting waveguide, the core may comprise aplurality of cores, and each of the cores is extended to adjacent onesoutside the end surface of the over cladding layer.

In the optical fiber mounting waveguide device, the optical fiber may bemounted in the optical fiber mounting groove, an end surface of theoptical fiber contacts with the core, in which the core and the opticalfiber are connected by curing an adhesive filled between the end surfaceof the optical fiber and end surfaces of the over cladding layer, thecore, and the under cladding layer, and a difference between refractiveindices of the core and the adhesive after curing is within ±0.005.

In the optical fiber mounting waveguide device, a material of the coremay be same as a material of the adhesive.

According to a third feature of the invention, a method for fabricatingan optical fiber mounting waveguide device comprises:

forming an optical fiber mounting groove on the optical fiber mountingwaveguide device for mounting an optical fiber;

forming an under cladding layer and a core sequentially on a substrateat a position facing to the optical fiber mounting groove;

coating a cladding material on the under cladding layer and the core;

providing a mask pattern for covering a part of the over cladding layerat the position facing to the optical fiber mounting groove; and

exposing the cladding material to provide an over cladding layer,

in which the core and the under cladding layer are protruded toward theoptical fiber mounting groove with respect to an end surface of the overcladding layer.

According to a fourth feature of the invention, a method for fabricatingan optical fiber mounting waveguide device comprises:

forming an optical fiber mounting groove on the optical fiber mountingwaveguide device for mounting an optical fiber;

forming an under cladding layer on a substrate at a position facing tothe optical fiber mounting groove;

coating a core material across the under cladding layer and a part ofthe substrate at a side of the optical fiber mounting groove;

directly exposing the core material to provide an L-shaped core whichcovers across the under cladding layer and a surface of the substrate atthe side of the optical fiber mounting groove;

coating a cladding material on the under cladding layer and the core;

providing a mask pattern for covering a part of the over cladding layerat the position facing to the optical fiber mounting groove; and

exposing the cladding material to provide an over cladding layer,

in which the core is protruded toward the optical fiber mounting groovewith respect to an end surface of the over cladding layer.

In the method for fabricating an optical fiber mounting waveguidedevice, the core may comprise a plurality of cores coupled to each otheroutside the end surface of the over cladding layer.

In the method for fabricating an optical fiber mounting waveguidedevice, the core may comprise a plurality of cores, and each of thecores is extended to adjacent ones outside the end surface of the overcladding layer.

The method for fabricating an optical fiber mounting waveguide devicemay further comprise:

mounting the optical fiber in the optical fiber mounting groove suchthat an end surface of the optical fiber contacts with the core; and

curing an adhesive filled between the end surface of the optical fiberand end surfaces of the over cladding layer, the core, and the undercladding layer, to connect the core and the optical fiber,

in which a difference between refractive indices of the core and theadhesive after curing is within ±0.005.

In the method for fabricating an optical fiber mounting waveguidedevice, the core material may be same as a material of the adhesive.

EFFECTS OF THE INVENTION

According to the present invention, it is possible to provide followingexcellent effects.

(1) The optical connection loss is small.

(2) The productivity is high.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, preferred embodiments according to the present invention will beexplained in conjunction with appended drawings, wherein:

FIGS. 1A to 1C are explanatory diagrams showing an optical fibermounting waveguide device in a first preferred embodiment according tothe present invention, wherein FIG. 1A is a plan view of the opticalfiber mounting waveguide device, FIG. 1B is a plan view of the opticalfiber mounting waveguide device on which an optical fiber is mounted,and FIG. 1C is a cross sectional view along A-A of the optical fibermounting waveguide device shown in FIG. 1B;

FIGS. 2A to 2C are explanatory diagrams showing an optical fibermounting waveguide device in a second preferred embodiment according tothe present invention, wherein FIG. 2A is a plan view of the opticalfiber mounting waveguide device, FIG. 2B is a plan view of the opticalfiber mounting waveguide device on which an optical fiber is mounted,and FIG. 2C is a cross sectional view along A-A of the optical fibermounting waveguide device shown in FIG. 2B;

FIGS. 3A and 3B are explanatory diagrams showing an optical fibermounting waveguide device in a fourth preferred embodiment according tothe present invention, wherein FIG. 3A is a plan view thereof, and FIG.3B is a cross sectional view thereof;

FIG. 4 is a plan view of an optical fiber mounting waveguide device in afirst conventional example;

FIG. 5 is a partially enlarged view of the conventional optical fibermounting waveguide device shown in FIG. 4;

FIG. 6 is a cross sectional view of an optical fiber mounting waveguidedevice in a second conventional example;

FIG. 7 is a plan view of an optical fiber mounting waveguide device in athird preferred embodiment according to the present invention;

FIGS. 8A to 8D are explanatory diagrams showing a method for fabricatingthe optical fiber waveguide mounting device in the first preferredembodiment according to the' present invention; and

FIGS. 9A to 9E are explanatory diagrams showing a method for fabricatingthe optical fiber waveguide mounting device in the second preferredembodiment according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Next, preferred embodiments according to the present invention will beexplained in more detail in conjunction with the appended drawings.

(First Preferred Embodiment)

FIGS. 1A to 1C are explanatory diagrams showing an optical fibermounting waveguide device in a first preferred embodiment according tothe present invention, wherein FIG. 1A is a plan view of the opticalfiber mounting waveguide device, FIG. 1B is a plan view of the opticalfiber mounting waveguide device on which an optical fiber is mounted,and FIG. 1C is a cross sectional view along A-A of the optical fibermounting waveguide device shown in FIG. 1B.

As shown in FIGS. 1A and 1B, an optical fiber mounting waveguide device1 in a first preferred embodiment according to the present inventioncomprises an under cladding layer 3 u as a first layer of a cladding 3,a core 4, and an over cladding layer 3 o as a second layer of thecladding 3 sequentially formed on the substrate 8. Further, an opticalfiber mounting groove 2 for mounting an optical fiber 6 is formed on theoptical fiber mounting waveguide device 1. In the optical fiber mountingwaveguide device 1, an end surface 3 a of the over cladding layer 3 ofaces to the optical fiber mounting groove 2 and a lateral groove 2 a,and the core 4 and the under cladding layer 3 u are protruded withrespect to the end surface 3 a of the over cladding layer 3 o at anoptical fiber mounting groove side, namely, toward the optical fibermounting groove 2.

The cladding 3 (the under cladding layer 3 u and the over cladding layer3 o) and the core 4 constitute an optical waveguide element. Aninterface between an optical waveguide element side and the opticalfiber groove side is indicated as a boundary 4 d.

The optical fiber mounting waveguide device 1 in the first preferredembodiment comprises four cores 4 provided in parallel with a constantpitch, and optical fiber fastening wedges 7 are formed on extended linesof central axes of regions between the respective cores 4, to provide aplurality of the optical fiber mounting grooves 2.

In the optical fiber mounting waveguide device 1 in the first preferredembodiment, the under cladding layer 3 u is provided directly beneath aprotruded part 4 a of the core 4, and the protruded part 4 a isprotruded with respect to the end surface 3 a of the over cladding layer3 o, namely with respect to the boundary 4 d.

(Method for Fabricating the Optical Fiber Mounting Waveguide Device 1)

Next, the optical fiber mounting waveguide device 1 in the firstpreferred embodiment will be explained below.

The optical fiber mounting waveguide device 1 is fabricated by thedirect exposure.

FIGS. 8A to 8D are explanatory diagrams showing a method for fabricatingthe optical fiber waveguide mounting device in the first preferredembodiment according to the present invention.

As shown in FIG. 8A, a UV curing cladding material is applied on thesubstrate 8 by using a spin coating machine, and UV irradiation isconducted to cure the UV curing cladding material, so as to form theunder cladding layer 3 u at a desired part. Thereafter, the UV curingcladding material at an unexposed part is removed by a developing fluid.

Next, as shown in FIG. 8B, a core material 4′ is applied on the undercladding layer 3 u and the direct exposure is conducted. Then, the corematerial 4′ at an unexposed part is removed by the developing fluid, toprovide the core 4.

Further, as shown in FIG. 8C, an over cladding material 3 o′ for theover cladding layer 3 o is applied on the core 4 and a photomask 10 forforming the protruded part 4 a of the core 4 is provided. Thereafter,the direct exposure is conducted.

As shown in FIG. 8D, the unexposed part is removed by the developingfluid, so that the exposed part remains as the cladding layer 3 o.

According to the aforementioned process, as shown in FIG. 1C, the core 4and the over cladding layer 3 o are formed such that the protruded part4 a of the core 4 and the under cladding layer 3 u are protruded to thesame extent from the end surface 3 a of the over cladding layer 3 o atthe side of the optical fiber mounting groove 2.

As shown in FIG. 1B, in the optical fiber mounting waveguide device 1,the optical fiber 6 is installed in the optical fiber mounting groove 2,and an end surface 6 a of the optical fiber 6 contacts with a tipportion (end surface) 4 a 1 of a protruded part 4 a of the core 4 and atip portion 3 b of the under cladding layer 3 u that are protruded withrespect to the end surface 3 a of the over cladding layer 3 o. A gapbetween the end surface 6 a of the optical fiber 6 and the end surface 3a of the over cladding layer 3 o, the tip portion (end surface) 4 a 1 ofthe protruded part 4 a of the core 4, and an end surface 3 b of theunder cladding layer 3 u is filled with an adhesive 9 having a samerefractive index as that of the core 4. The adhesive 9 is cured afterfilling, and it is sufficient if the refractive index of the adhesive 9is the same as that of the core 4 after curing.

(Function and Effect)

Next, function and effect of the present invention will be explainedbelow.

In the optical fiber mounting waveguide device 1 shown in FIGS. 1A to1C, when the optical fiber 6 is mounted in the optical fiber mountinggroove 2, the end surface 6 a of the optical fiber 6 contacts with thetip portion 4 a 1 of the protruded part 4 a of the core 4 that isprotruded with respect to the end surface 3 a of the over cladding layer3 o. Since the optical fiber mounting waveguide device 1 is fabricatedby direct exposure and the tip portion 4 a 1 of the protruded part 4 aprotruded with respect to the end surface 3 a of the over cladding layeris located at an edge of the photomask 10 (shown in FIG. 8C), the tipportion 4 a 1 of the protruded part 4 a of the core 4 has a roundnesswhen observed in an enlarged view thereof.

However, the present invention is different from the conventional devicein that the protruded part 4 a of the core 4 is protruded with respectto the end surface 3 a of the over cladding layer 3 o. Therefore, nocladding is provided between a curved surface of the tip portion 4 a 1of the protruded part 4 a and the end surface 6 a of the optical fiber6. Accordingly, when the adhesive 9 is filled between the end surface 3a of the over cladding layer 3 o and the end surface 6 a of the opticalfiber 6, the adhesive 9 intrudes between the curved surface of the tipportion 4 a 1 of the protruded part 4 a and the end surface 6 a of theoptical fiber 6.

As shown in FIG. 1B, a gap between the end surface 3 a of the overcladding layer 3 o and an end surface 7 a of the optical fiber fasteningwedge 7 is also filled with the adhesive 9.

The refractive index of the core 4 is equal to that of the adhesive 9after curing, so that a light propagates from the core 4 to the opticalfiber 6 or from the optical fiber 6 to the core 4 straightly through aconnecting portion between the core 4 and the optical fiber 6.Therefore, it is possible to prevent the light from leaking into theadjacent cores, thereby remarkably reducing the deterioration in theoptical isolation and the optical connection loss.

According to the above structure, it is possible to reduce adeterioration amount of the optical coupling depending on the relativeposition of the core with the optical fiber. Therefore, it is possibleto improve a mounting tolerance of the optical fiber, thereby improvingthe productivity of the optical fiber mounting waveguide device.

Further, in the first preferred embodiment, the under cladding layer 3 uis provided directly beneath the protruded part 4 a protruded withrespect to the end surface 3 a of the over cladding layer 3 o. Althoughthe under cladding layer 3 u is not indispensable, a mechanical strengthof the protruded part 4 a of the core 4 is reinforced by the undercladding layer 3 u. Therefore, when the optical fiber 6 is mounted inthe optical fiber mounting groove 2 and pressed to abut against theprotruded part 4 a of the core 4, a margin of a relative strength of anindenting pressure can be increased since a resistance characteristicfor the indenting pressure is increased. As a result, since theindenting pressure may be roughly adjusted, it is possible to improvethe productivity of the optical fiber mounting waveguide device 1.

According to the method for fabricating the optical fiber mountingwaveguide device 1 in the first preferred embodiment, since it ispossible to directly fabricate the core 4 having the protruded part 4 athat is protruded with respect to the end surface 3 a of the overcladding layer 3 o, the core 4 will not be damaged during themanufacturing process, thereby improving the productivity of the opticalfiber mounting waveguide device 1. In addition, since an excessiveprocess such as a process of removing the cladding is not required, itis possible to fabricate the optical fiber mounting waveguide device inlow cost.

(Second Preferred Embodiment)

Next, the second preferred embodiment according to the invention will beexplained below.

FIGS. 2A to 2C are explanatory diagrams showing an optical fibermounting waveguide device in a second preferred embodiment according tothe present invention, wherein FIG. 2A is a plan view of the opticalfiber mounting waveguide device, FIG. 2B is a plan view of the opticalfiber mounting waveguide device on which an optical fiber is mounted,and FIG. 2C is a cross sectional view along A-A of the optical fibermounting waveguide device shown in FIG. 2B.

As shown in FIGS. 2A to 2C, in the optical fiber mounting waveguidedevice 1, the core 4 is protruded with respect to the end surface 3 a ofthe over cladding layer 3 o, and formed downwardly until a bottomsurface 2 a 1 of the lateral groove 2 a to have an L-shape. The bottomsurface 2 a 1 of the lateral groove 2 a is provided at the same plane asthe bottom surface 2 b of the optical fiber mounting groove 2. In otherwords, a part of the under cladding layer 3 u in FIG. 1B is provided asa downwardly protruded part 4 b of the core 4.

(Method for Fabricating the Optical Fiber Mounting Waveguide Device 1)

Next, the method for fabricating the optical fiber mounting waveguidedevice 1 in the second preferred embodiment will be explained.

FIGS. 9A to 9D are explanatory diagrams showing a method for fabricatingthe optical fiber waveguide mounting device in the second preferredembodiment according to the present invention.

As shown in FIG. 9A, similarly to the first preferred embodiment, theunder cladding layer 3 u is formed on the substrate 8.

Next, as shown in FIG. 9B, a core material 4′ is applied on the undercladding layer 3 u and a part 8 a of the substrate 8 facing to theoptical fiber mounting groove 2.

Then, as shown in FIG. 9C, a photomask 11 for providing the protrudedpart 4 a and the downwardly protruded part 4 b of the core 4 is providedat a desired part of the core material 4′. Further, the direct exposureis conducted, and the core material 4′ at an unexposed part is removedby the developing fluid.

Further, as shown in FIG. 9D, a cladding material 3 o′ for the overcladding layer 3 o is applied on the core 4 and a photomask 10 forforming the protruded part 4 a of the core 4 is provided. Thereafter,the direct exposure is conducted.

As shown in FIG. 9E, the unexposed part is removed by the developingfluid, so that the exposed part remains as the cladding layer 3 o.

According to the above process, as shown in FIG. 2C, the protruded part4 a of the core 4 protrudes toward the optical fiber mounting groove 2with respect to the end surface 3 a of the over cladding layer 3 o, andthe downwardly protruded part 4 b of the core 4 reaches the bottomsurface 2 a 1 of the lateral groove 2 a, so that a total configurationof the core 4 is L-shaped. The over cladding layer 3 o is formed suchthat the protruded part 4 a that is a part of the L-shaped core 4protrudes with respect to the end surface 3 a of the over cladding layer3 o.

As shown in FIG. 2C, An end surface 4 b 1 of the downwardly protrudedpart 4 b contacts with the end surface 6 a of the optical fiber 6.

(Function and Effect of the Second Preferred Embodiment)

In the second preferred embodiment, the downwardly protruded part 4 b ofthe core 4 is provided directly beneath the protruded part 4 a, so thatmechanical strength of the protruded part 4 a of the core 4 isreinforced by the downwardly protruded part 4 b. Accordingly, it ispossible to improve the yield of the optical fiber mounting waveguidedevice. In addition, since the margin of the relative strength of theindenting pressure for the optical fiber is increased, it is possible toimprove the productivity of the optical fiber mounting waveguide device.

According to the above structure, it is possible to reduce thedeterioration amount of the optical coupling depending on the relativeposition of the core with the optical fiber. Therefore, it is possibleto improve a mounting tolerance of the optical fiber, thereby improvingthe productivity of the optical fiber mounting waveguide device.

(Third Preferred Embodiment)

FIG. 7 is a plan view of an optical fiber mounting waveguide device in athird preferred embodiment according to the present invention.

In the third preferred embodiment as shown in FIG. 7, a protruded part 4a of the core 4 may be extended in a lateral direction toward theadjacent cores and outside the end surface 3 a of the over claddinglayer 3 o, to provide an extended part 4 e. According to thisembodiment, even when the extended part 4 e of the protruded part 4 a ofthe core 4 has the roundness, an area of the end surface of the core 4contacting with the optical fiber 6 is increased. Accordingly, whenconnecting the optical fiber 6 with the core 4 by abutting the opticalfiber 6 and the core 4 to each other, the stable connection can berealized. Therefore, an excessive load will not be applied to theboundary (interface) 4 d between the end surface 3 a of the overcladding layer 3 o and the protruded part (core tip portion) 4 a,thereby reinforcing the mechanical strength of the device.

(Fourth Preferred Embodiment)

FIGS. 3A and 3B are explanatory diagrams showing an optical fibermounting waveguide device in a fourth preferred embodiment according tothe present invention, wherein FIG. 3A is a plan view thereof, and FIG.3B is a cross sectional view thereof.

In the fourth preferred embodiment shown in FIG. 3A, a plurality of thecores 4 are provided and the adjacent cores 4 are coupled to each otherby an lateral core 4 c provided outside the end surface 3 a of the overcladding layer 3 o.

In other words, the lateral core 4 c is provided across the protrudedparts 4 a of the cores 4.

In the fourth preferred embodiment, the protruded parts 4 a protrudedwith respect to the end surface 3 a of the over cladding layer 3 o arecoupled to each other by means of the lateral core 4 c, the mechanicalstrength of the core 4 a is further reinforced. At this time, a part ofthe lateral core 4 c may be provided inside the cladding 3.

Herein, FIG. 3B shows an example of the location of the lateral core 4c. The lateral core 4 c may be provided between the protruded parts 4 aof the core 4.

EXAMPLE

In the first to fourth preferred embodiments, GI50 (a core diameter of50 μm, and an optical fiber diameter of 125 μm) is used as the opticalfiber 6, an acrylic resin (a refractive index of 1.52) is used as thecladding 3 of the optical waveguide element, an acrylic resin (arefractive index of 1.57) is used as the core 4, and an epoxy resin (arefractive index of 1.57) is used as the adhesive 9. In addition, a filmthickness of the under cladding layer 3 u is 45 μm, the core 4 hasdimensions of 35 μm×35 μm, and a film thickness of the over claddinglayer 3 o at a region directly above the core 4 is 45 μm.

Since a bonding force of an epoxy adhesive is stronger than that of anacrylic adhesive, it is preferable to use the epoxy adhesive for theadhesive 9. Alternatively, when the acrylic adhesive is used for theadhesive 9, it is possible to easily realize the refractive indexmatching between the core 4 and the adhesive 9, since the material sameas the material of the core 4 is used as the adhesive 9. However, in thepresent invention, the material of the adhesive 9 is not limitedthereto.

Important factors for the adhesive 9 are as follows:

1) the material is transparent with respect to a wavelength of a lightto be used;

2) a difference between the refractive index of the material aftercuring and the refractive index of the core 4 is within ±0.005, morepreferably, the refractive index of the material after curing is equalto the refractive index of the core 4; and

3) the material has a sufficient bonding property with the optical fiber6 and the optical fiber mounting waveguide device 1.

It is possible to decrease the optical connection loss with the opticalfiber 6, when the difference between the refractive index of thematerial after curing and the refractive index of the core 4 is within±0.005.

A protruding length of the protruded part 4 a of the core 4 ispreferably from 10 to 50 μm. 10 μm is determined as a lower limit, sincethis length is a limit for a location shifting of the photomask in thedirect exposure, and the protruded part 4 a having a length greater than10 μm can be manufactured with an excellent accuracy. When theprotruding length of the protruded part 4 a of the core is around 70 μm,the optical connection loss is increased. However, when the protrudinglength of the protruded part 4 a of the core 4 is not greater than 50μm, the light will not be dispersed to be broader than the core diameterof the optical fiber, so that the increase in the optical connectionloss may be negligibly small.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be therefore limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. An optical fiber mounting waveguide devicecomprising: a substrate; an optical fiber mounting groove provided onthe substrate for mounting an optical fiber; an under cladding layer anda core sequentially formed on the substrate; and an over cladding layerformed on the core, the over cladding layer having an end surface facingto the optical fiber mounting groove, and wherein the core has aprotruded part extending toward the optical fiber mounting groove withrespect to the end surface of the over cladding layer and a downwardlyprotruded part.
 2. The optical fiber mounting waveguide device accordingto claim 1, wherein the core comprises a plurality of cores coupled toeach other outside the end surface of the over cladding layer.
 3. Theoptical fiber mounting waveguide device according to claim 1, whereinthe optical fiber is mounted in the optical fiber mounting groove, anend surface of the optical fiber contacts with the core, wherein thecore and the optical fiber are connected by curing an adhesive filledbetween the end surface of the optical fiber and end surfaces of theover cladding layer, the core, and the under cladding layer, and whereina difference between refractive indices of the core and the adhesiveafter curing is within ±0.005.
 4. The optical fiber mounting waveguidedevice according to claim 3, wherein a material of the core is same as amaterial of the adhesive.
 5. A method for fabricating an optical fibermounting waveguide device, comprising: forming an optical fiber mountinggroove on the optical fiber mounting waveguide device for mounting anoptical fiber; forming an under cladding layer on a substrate at aposition facing to the optical fiber mounting groove; coating a corematerial across the under cladding layer and a part of the substrate ata side of the optical fiber mounting groove; directly exposing the corematerial to provide an L-shaped core which covers across the undercladding layer and a surface of the substrate at the side of the opticalfiber mounting groove; coating a cladding material on the under claddinglayer and the core; providing a mask pattern for covering a part of theover cladding layer at the position facing to the optical fiber mountinggroove; and exposing the cladding material to provide an over claddinglayer, wherein the core is protruded toward the optical fiber mountinggroove with respect to an end surface of the over cladding layer.
 6. Themethod for fabricating an optical fiber mounting waveguide deviceaccording to claim 5, wherein the core comprises a plurality of corescoupled to each other outside the end surface of the over claddinglayer.
 7. The method for fabricating an optical fiber mounting waveguidedevice according to claim 5, wherein the core comprises a plurality ofcores, and each of the cores is extended to adjacent ones outside theend surface of the over cladding layer.
 8. The method for fabricating anoptical fiber mounting waveguide device according to claim 5, furthercomprising: mounting the optical fiber in the optical fiber mountinggroove such that an end surface of the optical fiber contacts with thecore; and curing an adhesive filled between the end surface of theoptical fiber and end surfaces of the over cladding layer, the core, andthe under cladding layer, to connect the core and the optical fiber,wherein a difference between refractive indices of the core and theadhesive after curing is within ±0.005.
 9. The method for fabricating anoptical fiber mounting waveguide device according to claim 8, whereinthe core material is same as a material of the adhesive.